Assembly systems, especially retractable assemblies, which are also referred to as push rod assemblies, are widely distributed in analytical measurements technology. They serve to introduce or withdraw measuring probes, e.g. sensors, into a process without interrupting the process. For this, the measuring probe is usually accommodated in a tubular holder, which is shiftably accommodated within an assembly housing of the assembly device and is guided between a measuring position and a maintenance position. The shifting of the tubular holder with the measuring probe can occur manually or automatically, for example, with the assistance of a pneumatic drive system. A pneumatic drive system for shifting the holder for the measuring probe can comprise a pneumatic cylinder and a movable, compressed air driven, piston therein, with which the tubular holder for the measuring probe is connected directly or via one or more connecting parts.
The assembly housing has a connection system, for example, a flange, to connect the assembly device to a process container. A calibration and/or cleaning chamber, into which the measuring probe can be drawn to a maintenance position, can be provided within the assembly housing in order to clean and/or calibrate the measuring probe.
In order to seal off the calibration and/or cleaning chamber from the process to be monitored, internal sealing rings are frequently arranged on the guide of the tubular holder in the region of the calibration chamber in the assembly housing; the sealing rings lie against smooth sealing surfaces on the process side end of the tubular holder.
Here and in the following, the side of the assembly device, on which the connection system for connecting to a process container is placed, is referred to as “process side”. Correspondingly, the direction along the central longitudinal axis of the assembly device toward the connection system is referred to as “on the process side”. The direction set counter to the process side direction is referred to as “facing away from the process.”
The firm of Endress+Hauser offers such assembly systems under the “Cleanfit” mark, examples of these are CleanFit S, CPA 471, CPA 472 and 472D, CPA 473, CPA 474 and CPA 475. The movement of the holder for the measuring probe occurs in an axial direction in these assembly systems, i.e. along a central axis of the assembly device. The movement occurs via a pneumatic drive automatically or by means of a manual actuation. In order to achieve a smooth approach of the retractable assembly to the particular position and especially to avoid damage to the measuring probe by too abrupt stopping in the measuring position or maintenance position, the axial movement of the measuring probe holder is damped with the assistance of a supplemental throttle element, which is mounted in the air supply of the pneumatic drive of the respective desired movement direction, namely in the measuring position or the maintenance position in assemblies with pneumatic drive systems.
As a supplemental component, such a throttle element is, however, on one hand, relatively expensive, especially because a throttle element is required for each movement direction, and on the other hand, has the disadvantage that it is only suitable in connection with the pneumatic drive for damping the movement of the measuring probe or the measuring probe holder.